Carrier Lifetime Measurements in LongWave Infrared InAsGaSb Superlattices Under Low Excitation

DOI:10.1007/s116-012-2216-1Ó2012TMS

CarrierLifetimeMeasurementsinLong-WaveInfraredInAs/GaSbSuperlatticesUnderLowExcitationConditions

DINGWANG,1,2DMITRIDONETSKY,1SEUNGYONGJUNG,1andGREGORYBELENKY11.—DepartmentofElectricalandComputerEngineering,StonyBrookUniversity,StonyBrook,NY11794,USA.2.—e-mail:wangding1985@gmail.com

Minoritycarrierlifetimeinlong-waveinfrared(LWIR)typeIIInAs/GaSbsuperlatticeswasstudiedusingtheopticalmodulationresponse(OMR)techniqueinwiderangesofexcitationandtemperature.Themeasuredcarrierlifetimewasfoundtoincreasesuperexponentiallywithdecreasingexcitationpowerdensitybelowthelevelof1mW/cm2to2mW/cm2.Thephenomenonwasqualitativelyexplainedbythepresenceofshallowtrappingcenters.Keywords:Carrierlifetime,long-waveinfrared,superlattice

INTRODUCTION

Antimony-basedtypeIIstrained-layersuperlat-tices(T2-SLSs)areofgreatinterestinthedevel-opmentofhigh-performanceLWIRphotodetectorsandfocal-planearraysduetotheirbandgaptun-ability,materialuniformity,andlowAugerrecom-bination.1–3Inthepastseveralyears,majoreffortshavebeenmadetoimprovetheperformanceofT2-SLSphotodetectorsregardinggrowth,processing,andstructuraldesign.4–7CurrentdesignsutilizeapÀ-dopedabsorberandheterojunctionbarrierstoreducethegeneration–recombinationandtunnelingcomponentsofdarkcurrent.6TheperformanceofsuchphotodetectorswasreportedtobelimitedbytheShockley–Read–Hall(SRH)carrierrecombina-tionprocessintheabsorber.8WepreviouslyreportedthemeasurementofminoritycarrierlifetimeinaLWIRT2-SLSstruc-turewithenergygapof0.12eVat77K.9TheSRH-limitedlifetimeof30nsat77KwasdeterminedfromthedependenceoflifetimeonexcitationpowerusingtheOMRtechnique.SimilarlifetimevaluesweredeterminedfromdiodeI–Vcharacteristics10andtime-resolvedphotoluminescence(TRPL)decaymeasurements.11Inthisworkwepresenttheresultsofminoritycarrierlifetimemeasurementsperformedatlowerexcesscarrierdensities(approximatelyfrom1014cmÀ3to1015cmÀ3)inawidertemperature

(ReceivedAugust1,2011;acceptedJuly25,2012;publishedonlineAugust28,2012)

range(from20Kto250K).Theresultsrevealinterestingfeaturesofthemechanismoftheminoritycarrierrecombinationprocessunderlowexcitationconditions.

EXPERIMENTALPROCEDURES

TheSLSstructurewasgrownbymolecularbeamepitaxyonalow-dopedp-typeGaSbsubstrate.Thestructureconsistsofa1.8-lm-thickInAs/GaSbSLSlayer(400periods)enclosedwithinthinAlSbAscarrierconfinementlayers.AthinGaSbcaplayerwasgrownontopofthestructuretopreventoxi-dation.TheSLSabsorberhad13monolayers(ML)ofundopedInAsand7MLofberyllium-dopedGaSb.Thetargetbackgroundholeconcentrationat77Kwas191016cmÀ3.Thestructureexhibitedaphotoluminescence(PL)emissionpeakat0.16eVat77K.

TheminoritycarrierlifetimewasmeasuredbytheOMRtechnique.9,12Opticalexcitationwasgen-eratedbyadirectlymodulated1.5-lmfiber-coupledlaserdiode.TheexcitationbeamwasconvergedbyaCaF2lens;thebeamwidthathalf-maximumwas380lmonthespecimensurface.Weusedareflec-tiveobjectivetocollectthePLwithinasolidangleofabout1.8steradians.ThePLwasconvergedbyaZnSelensontoa250-lm-diameterliquid-nitrogen-cooledmercurycadmiumtelluride(MCT)photode-tector.ThemodulatedPLsignalwasselectedandamplifiedbyaStanfordResearchradiofrequency(RF)lock-inamplifier(SR844).Thenarrow-band

3027

3028amplificationofthesine-wavesignalbythelock-inamplifierimprovedthesignal-to-noiseratio,allowingforthereductionoftheopticalexcitation.TwoparameterswereextractedfromthefitofthePLfrequencyresponsetothedependencePLx/[1+(xs)2]0.5:thecarrierlifetimes,andtheamplitudeofthemodu-latedPLsignalinthelow-frequencylimit(x«1/s),PLthemodulationamplitudeoftheexcitationx.Whenissmallcomparedwiththesteady-stateexcitation,thederivativeofthesteady-statePLintensityPLrespecttothesteady-statecarriergenerationrate0withGbereplacedbytheratioofthemodulationamplitudes0canPLx/G1,whereG1istheamplitudeofthemodulatedexcitationintermsofthecarriergenerationrate.Therefore,PLxcanbeexpressedas

PLx¼

dPL0

dGG1

ðx(1=s;G0

1(G0Þ:

(1)

G0andG1werecalculatedfromtheexcitationpowerandareaassumingcompleteabsorptionoftheopticalexcitationinthesuperlatticelayer.Thereflectionof35%oftheexcitationpowerattheinterfacebetweenthesampleandairwastakenintoaccount.Theexcitationpowerwascontrolledbyeitherthelaseroperatingcurrentortheopticalattenuationusingavariablemetallicneutralden-sityfilter.

RESULTSANDDISCUSSION

Figure1apresentsthePLfrequencyresponsedatameasuredatT=20Kforaseriesofexcitationlevels.Thelifetimevaluederivedfromfittingwass=26nsatexcitationof13.6W/cm2,correspondingtoexcesscarrierconcentrationDn=691015cmÀ3orÀ3excesscarriergenerationrateG0=2.391023cmsÀ1.Arapiddecreaseofthereciprocalcarrierlifetime1/stowardslowexcitationwasobserved,asshowninFig.1b.Alifetimeofs=58nswasobtainedfromfittingatexcitationof0.23W/cm2.Thephenomenon

(a)20(b)515 mWT = 20 KT = 20 K)7.8 mWB4d e04.0 mW()1-ssn 37o1.9 mW0p1 se1.0 mWx( 2r L-20τ0.50 mW/1P10.26 mW-40100k1M10M01021102210231024Frequency (Hz)Carrier generation rate G-3s-10 (cm)Fig.1.(a)PLfrequencyresponsedataat20Katvarioussteady-stateexcitationpowersfrom0.26mWto152mW.EachresponsecurvewasfittedtothefunctionPLx/[1+(xs)]0.5.Theexcitationareawas1.1910À3cm2.(b)Dependenceofreciprocalcarrierlifetimeontheexcesscarriergenerationrateat20K.Wang,Donetsky,Jung,andBelenky

canbepossiblyassociatedwithashallow-trap-relatedrecombinationprocess,asdiscussedbelow.Inthesteady-statecondition,thegenerationandrecombinationratesoftheexcesscarriersareequal:

GDn0¼

s;(2)

r

wheresristherecombinationlifetimeofexcesscarriers.Previousresultshaveshownthat,inInAs/GaSbsuperlattices,theSRHprocessdominatestherecombinationofexcesscarriersforamoderatelevelofexcitation(Dn=1015%1016cmÀ3).12,13Themea-suredvaluesofs30ns,asrweretypicallyintherangefrom20nstoshowninFig.2(righty-axis)atG0>391022cmÀ3sÀ1(orDn>191015cmÀ3).Thesteady-stateminorityexcesscarriershaveaprobabilitytorecombineradiativelyandgeneratePL.Thesteady-statePLintensityinap-typematerialcanbewrittenas

PL0¼B0DnðDnþp0Þ;

(3)

whereB¢istheradiativerecombinationcoefficienttakingintoaccountthecollectionefficiencyofthedetectionsystem;p0isthebackgroundholecon-centrationatequilibrium.Theparameter(PLbecalculatedfromthederivativeofPLx/Gwith1)canrespecttoG00,combiningEqs.1–3.SinceÀs3risaslowfunctionofG0forG0>391022cmsÀ1,thederivativeofsrisomittedandtheexpressioncanbesimplifiedas

PLxG¼dPL0

%Bðp0þ2G1dG0

0srÞsrðx(1=s;G1(G0;dsr=dG0%0Þ:

ð4Þ

12060T = 20 K50)sn() .80τu.ea(m i1tG40ef/iωl LrP40eirr30aC0051015202520Carrier generation rate G22-3-10 ( x 10 cms)Fig.2.Plotof(PLx/G1)asafunctionofthesteady-stategenerationrateofexcesscarriersG0at20K(triangles,lefty-axis).PLxwasmeasuredatalowmodulationfrequencyatf=100kHz;G1wassetbelow5%ofG0duringthemeasurementtovalidateEq.1.Therighty-axisshowsthedependenceofcarrierlifetimeontheexcesscarriergenerationrateG0at20K(circles).ThegreydashedlineindicatesG0=391022cmÀ3sÀ1.CarrierLifetimeMeasurementsinLong-WaveInfraredInAs/GaSbSuperlattices3029

UnderLowExcitationConditions

Figure2(lefty-axis)showsthemeasured(PL91022cmÀ3sÀ1,(PLx/GG1)asafunctionof0.ForG0>3x/G1)wasnearlyconstant,probablyduetoarelativelylargebackgroundcarrierconcentration(p0»2Gtoalinearfunction0sr).Thus(PLx/G1)approximatespropor-tionaltotherecombinationlifetimesexcitations.However,whenther,especiallyatlowexcitationwasdecreasedtothelevelofG(PL0<391022cmÀ3sÀ1,asshowninFig.2,x/G1)droppedrapidlywhilethemeasuredlifetimeincreasedsuperexponentially.Thisdiscrepancycanbepossiblyattributedtothepresenceofshallowtraps.

Shallow-trap-basedmodelshavecommonlybeenusedtoexplaintheabnormallyhigheffectivelifetimesderivedbyphotoconductancemeasurementsunderlowinjectioninSi.13,14Trappingofminoritycarrierssup-pressestherecombinationofexcesscarriersthroughSRHcentersandresultsinalargerphotoconductanceand,therefore,anincreasedeffectivelifetime.Theprocessmayhaveasimilarimpactonthecarrierlife-timemeasuredbyopticaltechniques.15Intheconditionoflowcarrierinjection,i.e.,closetothedensityofshallowtraps,aconsiderableportionofthesteady-stateexcesselectronsinp-typematerialscanbecapturedbytheshallowtraps.Thecapturedelectronscanescapefromthetrapstotheconductionbandataslowerrate,dependingonthetemperature,theenergylevelsofthetraps,andotherfactors.Thisrecyclingprocesscandecreasetheoverallrateofelectronrecombinationthroughdeepcentersandcouldbethereasonfortheobservedincreaseofthemeasuredminoritycarrierlifetime.Statistically,sinceaconsiderableportionoftheexcesselectronsistrapped,lowPLintensityisalsoexpected.Oncethefluxdensityofexcitationphotonsisincreasedtoacertainlevel,mostoftheshallowtrapsareoccupiedbyelectrons,sothemeasuredlifetimewillreflecttheactualrecombinationrateoftheminorityelectrons.

1030925)s20τ ~ T-1/28n(15 τ107501002003001)-sT (K)7 601x(5 τ/143 T = 20 K2 T = 77 K T = 150 K10510152025G22-3-10 (x10 cms)Fig.3.DependenceofreciprocalcarrierlifetimeonexcesscarriergenerationrateG0atT=20K(circles),77K(triangles),and150K(squares).TheinsetplotstheminoritycarrierlifetimemeasuredatG0=2.391023cmÀ3sÀ1asafunctionoftemperature(dots).Thelineisthefitofthefunctions$TÀ1/2.Thesignal-to-noiseratioachievedinthisworkallowedcarrierlifetimemeasurementsinawidetemperaturerangeupto250K.Figure3presentsthedependenceofreciprocalcarrierlifetimeonexcesscarriergenerationrateatT=20K,77K,and150K.Asharpdecreaseof1/swasapparentupto77K.DeterminationofthecarrierlifetimeatlowexcitationwaslimitedbyPLquenchingatelevatedtemperatures.Thedependenceofmeasured1/2carrierlifetimesontemperaturefollowedTÀupto250Kinthemeasuredrangeofexcitation,whichcanbeattributedtothechangeofmeanthermalvelocityofexcesscarriers.TheinsetofFig.3showsthecarrierlifetimeasafunctionoftemperaturemeasuredatG0=2.391023cmÀ3sÀ1.

CONCLUSIONS

Wehavepresentedtheresultsoffrequency-domaincarrierlifetimemeasurementsinp-dopedLWIRT2-SLSunderlowexcitationconditions.Thelifetimevalueof58nswasobtainedatthelowexcitationlevelG0=391021cmÀ3sÀ1.Asuperexponentialincreaseofthemeasuredminoritycarrierlifetimewasfoundwhentheexcitationleveldecreasedbelow1mW/cm2to2mW/cm2.Aqualitativeexplanationbasedoncaptureofexcesselectronsbyshallowtrapswaspro-posed.Thecarrierlifetimewasfoundtofits$TÀ1/2inthetemperaturerangeupto250K.

ACKNOWLEDGEMENTS

TheauthorswouldliketothankDr.StefanP.Svenssonforhelpfuldiscussions.TheLWIRSLSstructureswereprovidedbyUSArmyNVESD.ThisresearchwassupportedbyUSArmyNVESDandtheNationalScienceFoundation(GrantDMR07101).

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